A magnetic sensor and a magnetic head, which utilize a magneto-resistance effect phenomenon that applying a magnetic field into a magnetic structure such as a spin valve structure causes a change in electric resistance, have been developed and used in practice.
In recent years, noticed having been a magnetic random access memory (MRAM) using such a magneto-resistance effect. In the magnetic random access memory, the electric current is let flow into the magnetic structure to correspond to digital information of “0” or “1” by using a fact that the resistance value is changed in accordance with a direction of a spin of an electron in a magnetic material.
In such an MRAM, a magnetic field generated by a current flow into a writing line is used for determining a direction of magnetization of a free layer, which is to be a writing layer, to write data in the vicinity of a memory cell. A structure of the memory cell, however, should be made minute in accordance with a recent request for higher density.
The current for generating a magnetic field necessary for inverting a direction of magnetization of a ferromagnet comprising the free layer should be several mA or more when the memory cell is made minute. Accordingly, the consumption power increases and the Joule heat is generated, so that problems occur. These are great causes preventing a memory from being made higher in density.
In order to solve such problems, proposed have been a current induction magnetic structure changing type MRAM in which the direction of magnetization of a free layer is inverted not by application of a magnetic field but by injection of a spin polarization electron into a free layer (Patent References 1 to 3, for example).
Such a current induction magnetic structure changing type MRAM has problems that necessity of a new mechanism for injecting a spin polarization electron into a free layer causes a complicated structure of a memory cell and that lowering in writing current is not always enough.
On the other hand, also proposed has been application of a phenomenon to a magnetic memory, the phenomenon that a flow of a current into a minute magnetic material causes a magnetic domain wall formed in the magnetic material to move (refer to Non-Patent Reference 1 or 2, for example).
Such a phenomenon that a flow of a current into a minute magnetic material causes a magnetic domain wall formed in the magnetic material to move has been known from a long time ago. It has been known that a localized magnetic domain wall behaves as a particle with inertia, namely, the pseudo mass “m” due to the angular momentum of spins.
A theoretical analysis relating to such a mechanism of movement of a magnetic domain wall has been proposed recently (refer to Non-Patent Reference 3, for example). Accordingly, the mechanism of movement of a magnetic domain wall will be described now, made reference to FIG. 20.
Refer to FIG. 20.
FIG. 20 is a conceptually perspective view of a mechanism of movement of a magnetic domain wall. The upper drawing shows a case that a current “I” is let flow into a magnet wire 81 in which a single magnetic domain wall is formed. A flow of the current “I” in the magnet wire 81 in an X direction from the left side causes a momentum of an electron to operate on a single magnetic domain wall 82 so that a magnetic moment of the single magnetic domain wall 82 would rise to move with torsion at an angel φ0 in a Z direction.
As a mechanism contributing to the movement in the above case, there are two mechanisms: one is momentum transfer with an electron; and one is electron angular momentum, namely, spin transfer.
The middle drawing in FIG. 20 illustrates the momentum transfer. A flow of the current “I” from the left corresponds to a flow of an electron 85 from the right. The electron 85 flowing from the right gives the single magnetic domain wall 82 momentum of the electron 85 when the electron 85 is scattered interactively with the single magnetic domain wall 82 by the spin of the electron 85. Accordingly, the single magnetic domain wall 82 is to move in a −X direction. Force FMT operating on the single magnetic domain wall 82 in the above case is proportional to a quantity of the electron 85, namely, the current “I”, and thereby, expressed by:FMT∝I.
The lower drawing in FIG. 20 illustrates the spin transfer. In the case that the current “I” flows from the left and the electron 85 passes over the single magnetic domain wall 82, the spin of an electron 86 flowing into a magnetic section 83 is accorded with a direction of the magnetic moment of the magnetic section 83. This causes the area of the magnetic section 83 to be increased, so that the single magnetic domain wall 82 moves in the X direction.
A moving amount ΔX for a minute time Δt in the above case is proportional to the spin amount of an electron, namely, a spin current IS. Accordingly,ΔX∝ISΔt,and in accordance with the above proportional relation,dX/dt=IS can be assumed.
In the above assumption, the force FST of the operation of the spin is secondary differential of the displacement amount X for time and expressed by:FST∝d2X/dt2∝dIS/dt.
In such current induction magnetic domain wall movement, an operation described with respect to the spin transfer can be considered to be superior since a size of the single magnetic domain wall 82 is nm in order and long enough for a sub nano-order of a wavelength of the electron 85. This does not contradict results of various kinds of experiments using a direct current or a pulse current (refer to Non-Patent Reference 4, for example).
Patent Reference 1: JP-A-2003-017782
Patent Reference 2: JP-A-2003-281705
Patent Reference 3: JP-A-2004-006774
Non-Patent Reference 1: Applied Physics Letters, Vol. 81, No. 5, pp. 862-864, 29 Jul. 2002
Non-Patent Reference 2: Nature, Vol. 428, pp. 539-542, 1 Apr. 2004
Non-Patent Reference 3: Physical Review Letters, Vol. 92, 086601, 27 Feb. 2004
Non-Patent Reference 4: Applied Physics Letters, Vol. 83, p. 509, 2003